In producing a magnetic recording medium such as a magnetic recording tape, a magnetic recording disc, etc., by a coating process, the magnetic recording medium is generally produced by coating a magnetic coating composition, prepared by dispersing a ferromagnetic powder in a solution of a high molecular binder, on a non-magnetic support and then drying the coating. If foreign matter such as a dispersion of magnetic powder, re-aggregation matter, resinous insoluble matter, etc., exists in the liquid supplying system just prior to coating, it causes dropout, reduction of output, increased noise, etc. therefore, it has been proposed to filter the coating liquid to remove foreign matter.
In FIG. 2, a magnetic coating composition 1, prepared by uniformly dispersing a ferromagnetic powder in an organic solvent solution of a binder, is introduced into a stock tank 2 and travels through a conduit 3 by means of a pump P. The coating composition is filtered by a filter 4 to remove foreign matter. When such a filter is used various conditions such as the nature of the material, the form, etc., of the filter and the viscosity, feeding pressure, flow amount, etc., of the magnetic coating composition are investigated.
To improve the filtering effect in the above-described apparatus, it has been proposed to stir the magnetic coating composition for a long period of time prior to filtering as described in, for example, Japanese Patent Application (OPI) No. 117,737/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
It has also been proposed to treat a magnetic coating composition with ultrasonic waves in an ultrasonic tank for from about several minutes to about several tens minutes to disperse undispersed matter, reaggregation matter, resinous insoluble matter, etc., before coating the coating composition as disclosed in, for example, Japanese Patent Publication No. 21,331/73 and Japanese Patent Application (OPI) No. 87,303/74.
European Patent Publication No. 85,138 discloses that when treating a magnetic coating composition by ultrasonic waves, if the coating composition is coated after circulating the composition for about 5 hours while applying the ultrasonic treatment that magnetic coating composition can be coated in a stable dispersed state without causing aggregation.
However, the following problems are encountered when using the above-described filter systems.
That is, when the mesh of a filter which is used directly before coating a magnetic coating composition is fine, the above-described problem of the magnetic coating composition at coating is small; but, if the mesh of the filter is too fine, the ferromagnetic powder or materials added for reinforcement are removed by the filtration, whereby the desired performance of the filtration sometimes cannot be obtained. Also, the filter is frequently clogged by magnetic powder, etc., and the filter must be frequently replaced which requires interuption of the coating machine each time. As described above, if the mesh of the filter for use in the filtration procedure is too fine, the production rate of magnetic recording medium is greatly reduced and the workability for the coating operation is very low.
However, in conventional systems which use a filter, the filter must be frequently replaced even when ignoring the production rate and workability to counteract the occurrence of dropout, the reduction of output, the increase of noise, etc. Accordingly, there has been an urgent need to develop a filtration technique capable of prolonging the time between replacing filters used directly before coating a coating composition without damaging the filtration operation.
In the above-described ultrasonic treatment, the problems that occur when using a filter (described above) do not occur since a filter is not used; but, the effects of using a filter are not obtained. To obtain a stabilized dispersion using ultrasonic treatment, a magnetic coating composition must be circulated for about 5 hours while applying the ultrasonic treatment before the magnetic coating composition is coated and, hence, it takes a considerably long period of time to perform the circulation and it is impossible to continuously perform the coating operation for a long period time.